Theoretical study of mode competition including backward wave and experiment on the 28 GHz second harmonic gyrotron

The gyrotron operates in the harmonic state, which can significantly reduce the required operating magnetic field but complicates the mode competition problem. In some situations, it is also possible for the backward wave of the fundamental wave mode to be excited to suppress the desired harmonic operating mode. Therefore, a detailed analysis of mode competition is necessary for gyrotrons operating in the harmonic state. This paper presents the theoretical research of mode competition and experimental work for the 28 GHz second harmonic gyrotron at the TE02 mode. Possible mode competition problems are analyzed in detail using the linear theory and the multimode time-dependent non-linear theory. The problem of excitation of the backward wave of the fundamental wave mode TE21, which may be present, is received focused investigation. Ultimately, the possible competition modes are successfully suppressed in the non-linear simulation by setting up a suitable startup scenario, and an interaction efficiency of 32.5% is obtained. Based on the theoretical research results, experimental verification is carried out. When the working voltage is 29.8 kV, the beam current is 1.18 A, and we obtain stable single mode operation of the gyrotron, which has an operation frequency of about 27.96 GHz and an interaction efficiency of about 30.1% from the experiment.